The present invention relates to fabrication methods for circuit boards, and more particularly, to a method for fabricating a substrate or a printed circuit board (PCB).
In conventional processes for fabricating a circuit board such as substrate or printed circuit board (PCB), first, a core layer is prepared by fiber glass, epoxy resin, polyimide, FR4 resin or BT resin, etc. Then, at least a copper film is attached to the core layer, and patterned to form a plurality of conductive traces on the core layer. Since the trace-forming process is well known in the art, it is not to be further described herein.
After that, solder mask is applied over the conductive traces to form protective coating on the core layer, so as to protect the conductive traces against oxidation or short circuit without affecting electricity thereof. In more detail, solder mask is first applied over the core layer by halftone-printing, roller-coating, screen-coating or electrostatic-spraying processes. Then, undesired part of solder mask is removed through the use of exposure and development techniques; remaining solder mask is baked under high temperature and become cured to form the protective coating.
However, the above conventional circuit board is inherent with significant drawbacks. For example, during the process for applying solder mask over the core layer, halftone-printing or roller-coating processes are performed in multiple times for accumulating solder mask with desired thickness; this considerably increases process complexity in fabrication. And, accumulated thickness of solder mask is hardly controlled, which may adversely affect planarity and electrical stability of the circuit board. Moreover, in the baking process under high temperature, due to mismatch in coefficient of thermal expansion (CTE) between solder mask and the core layer of the circuit board, thermal stress would be generated and causes warpage of the circuit board. Furthermore, during coating solder mask over the core layer, air would be possibly trapped in solder mask to form voids, which facilitates the occurrence of popcorn effect in subsequent fabrication processes. In addition, solder mask is poorly adhered with copper traces formed on the core layer; this would easily cause delamination at interface between solder mask and conductive traces, and undesirably affect quality and reliability of the circuit board.
Therefore, there is disclosed another method for fabricating a circuit board. This fabrication method for a circuit board 1 can be carried out by process steps illustrated in FIGS. 5A to 5E. Referring to FIG. 5A, the first step is to prepare a core layer 10, with predetermined patterning of copper conductive traces 11 being formed on two opposing surfaces of the core layer 10. A plurality of vias 12 are formed to penetrate through the core layer 10, and plated with copper on inner walls thereof The core layer 10 can be made of a material same as that used for a conventional circuit board, such as epoxy resin, polyimide resin, FR4 resin, etc.
Referring to FIG. 5B, the next step is to apply a non-solderable material 14 in predetermined thickness over an aluminum film 13. The non-solderable material 14 is preferably made of a material having coefficient of thermal expansion (CTE) similar to or same as that of the core layer 10.
Then, the non-solderable material 14 together with the aluminum film 13 are attached to the core layer 10 for covering the conductive traces 11, in a manner that the non-solderable material 14 is interposed between the aluminum film 13 and the core layer 10. Under predetermined pressure (10-40 kgw/cm2) and temperature (185xc2x0 C.), the non-solderable material 14 becomes cured and fully fills the vias 12 and other fine holes of the core layer 10, and thus the non-solderable material 14 forms a desired protective layer for protecting the conductive traces 11 against oxidation and external impact.
Referring to FIG. 5C, a layer of photo resist 15 is applied over an exposed surface of the aluminum film 12. The photo resist 15 is selectively removed by using exposure and development processes, so as to expose predetermined part of the aluminum film 13, wherein the exposed part of the aluminum film 13 corresponds to predetermined positions of the underneath conductive traces 11 to be later exposed for use as bond pads or fingers where solder balls, bumps or wires are bonded for electrical connection purpose.
Referring to FIG. 5D, remaining photo resist 15 and exposed part of the aluminum film 13 are etched away by using chemical solvents, so as to expose predetermined part of the non-solderable material 14 covering bond pad or finger positions of the underneath conductive traces 11. Then, the exposed part of the non-solderable material 14 is removed by plasma etching technique, such that bond pads or fingers of the conductive traces 11 can be desirably exposed.
Finally referring to FIG. 5E, remaining aluminum film 13 is chemically etched to completely expose the non-solderable material 14.
The above-fabricated circuit board 1 can desirably eliminate those outlined drawbacks for the foregoing conventional circuit board. For example, one single step of applying non-solderable material 14 allows to desirably achieve predetermined thickness for the non-solderable material 14, thereby effectively reducing complexity and costs in fabrication. Moreover, since the non-solderable material 14 has coefficient of thermal expansion similar to or same as that of the core layer 10, the circuit board II can be assured with structural intactness without being warped by thermal stress, thus making production yield greatly improved. In addition, the non-solderable material 14 is firmly attached to the core layer 10 and conductive traces 11 under condition of certain temperature and pressure, and thus air is hardly trapped in the non-solderable material 14, so that popcorn effect or delamination would significantly reduce in occurrence, making quality and reliability of the circuit board 1 firmly assured.
However, the above fabrication method for the circuit board 1 still has considerable drawbacks. First, conductive traces 11 formed on the core layer 10 are covered by multi-layered structure including the non-solderable material 14, aluminum film 13 and photo resist 15; as such, it is difficult to visually recognized predetermined part of the conductive traces 11 to be exposed through such multi-layer structure. In order to precisely position corresponding part of the photo resist 15 aligned with the part of the conductive traces 11 to be exposed, X-ray fluoroscopy is usually adopted to determine fiducial marks on the photo resist 15. However, X-ray fluoroscopy still possibly causes positioning inaccuracy up to xc2x175 xcexcm, making etched part of the aluminum film 13 and non-solderable material 14 not precisely positioned in correspondence with the predetermined exposed part of the conductive traces 11, which deteriorates production yield of fabrication circuit boards 1.
Moreover, during fabrication processes, with the photo resist 15, aluminum film 13 and non-solderable material 14 being stacked on the core layer 10, for exposing predetermined part of the conductive traces 1, it needs to in turn remove the photo resist 15, aluminum film 13 and non-solderable material 14, thereby making process complexity and costs in fabrication undesirably increased.
Furthermore, the non-solderable material 14 is partly removed by plasma-etching technique; for suitably applying the plasma-etching process, the non-solderable material e.g. epoxy resin 14 cannot be added with fillers, which makes the non-solderable material 14 relatively low in structural strength, thereby not able to provide the circuit board 1 with high rigidity or mechanical strength.
In addition, the plasma-etching technique is used to selectively remove the non-solderable material 14 and form a plurality of openings where predetermines positions of the conductive traces 11 can be exposed, wherein the exposed positions of the conductive traces 11 serve as bond pads or fingers for subsequently accommodating solder balls, bumps or wires for electrical connection purpose. These openings formed by the plasma-etching technique are SMD (solder mask define) openings; as shown in FIG. 5E, each SMD opening is dimensioned smaller in surface area than the underneath conductive trace 11; that is, part of the underneath conductive trace 11 is unexposed and covered under the non-solderable material 14. As such, adjacent openings are at least spaced apart from each other by the distance of unexposed part of adjacent conductive traces 11; this would hardly reduce the distance between adjacent openings and pitch spacing between neighboring bond pads or fingers, making the circuit board 1 not suitably used for mounting with fine-pitch or high-density arrangement of solder balls, bumps or wires.
During the process for attaching the non-solderable material 14 to the core layer 10, it is hard to precisely control the planarity of the non-solderable material 14 relative to that of the core layer 10. With unsatisfactory planarity of the non-solderable material 14, for example, certain part of the non-solderable material 14 would be relatively small in thickness, during the plasma-etching process, relatively thinner portion of the non-solderable material 14 would be completely etched away in accompany with part of or entire the underneath conductive traces 11 (only 15 xcexcm thick); this undesirably damages electrical connection quality and production yield of fabricated circuit boards 1.
As shown in FIG. 6, if the aluminum film 13 is surface-roughed or low in surface smoothness with forming of protrusions 16, these protrusions 16 extend through the non-solderable material 14 and are hard to be removed completely by etching technique. If the protrusions 16 even extend to reach the conductive traces 11, it may cause short-circuiting problems. For improving the planarity of the aluminum 13, it would nevertheless increase fabrication costs.
As shown in FIG. 7A, another drawback for the above circuit board 1 is side-etching effect. During the plasma-etching process for partly removing the non-solderable material 14, plasma-produced radicals are non-directional and may also etch side walls of openings formed through the aluminum 13 and non-solderable material 14, i.e. side-etching effect. This side-etching effect would become more severe downwardly toward the non-solderable material 14, and generate more adverse influence on positioning or dimensioning of the openings for exposing bond pad or finger positions of the conductive traces 11. As shown in FIG. 7B, when solder balls (as indicated by dotted lines in the drawing) are implanted at these side-etched openings, since solder balls and the non-solderable material 14 are different in coefficient of thermal expansion, thermal stress produced by the non-solderable material 14 would attack solder balls (as indicated by arrows in the drawing), and impair electrical connection between solder balls and the circuit board 1 that is thereby of quality and reliability concern.
Therefore, how to develop a novel fabrication method for a circuit board to eliminate the above drawbacks, is a critical problem to solve.
A primary objective of the present invention is to provide a fabrication method for a circuit board, so as to precisely expose predetermined positions (bond pads or bond fingers) of conductive traces formed on the circuit board, thereby greatly improving yield of fabricated circuit boards.
Another objective of the invention is to provide a fabrication method for a circuit board, wherein a support member, instead of an aluminum film conventionally used in the art, can be repetitively used to sustain a non-solderable material thereon, and easily peeled from the non-solderable material, making fabrication time and production costs effectively reduced.
A further objective of the invention is to provide a fabrication method for a circuit board, wherein conventional plasma etching technique is not adopted for fabricating the circuit board, so that drawbacks of side-etching effect and impaired electrical connection through the use of plasma etching technique can be eliminated, thereby assuring quality and reliability of fabricated circuit boards.
A further objective of the invention is to provide a fabrication method for a circuit board, which allows the circuit board to be optionally made with relatively soft or rigid structure, thereby providing more flexibility in modulating mechanical strength of fabricated circuit boards.
In accordance with the above and other objectives, the present invention proposes a fabrication method for a circuit board, comprising the steps of: preparing a core layer that is formed with predetermined circuitry of conductive traces on at least a surface thereof, each of the conductive traces being formed with a terminal, and applying photo resist on the terminal of each of the conductive traces; peelably applying a non-solderable material in predetermined thickness over a support member, and attaching the non-solderable material to the surface of the core layer where the conductive traces are formed, wherein adhesion between the support member and the non-solderable material is smaller than adhesion between the non-solderable material and the core layer; heating and pressurizing the non-solderable material, allowing the non-solderable material to be firmly attached to the core layer; peeling the support member from the non-solderable material to expose the non-solderable material; partly removing the non-solderable material to expose the photo resist applied on the core layer; and removing the photo resist from the core layer to expose the terminals of the conductive traces.
The above-fabricated circuit board is suitably used as a substrate for accommodating integrated circuits, or a printed circuit board (PCB); and the circuit board can provide significant benefits. First, terminals (bond pads or fingers) of conductive traces formed on a core layer of the circuit board are in advance covered by photo resist, and then directly exposed by removing the photo resist. This assures terminal positions of the conductive traces to be precisely exposed, and therefore, eliminates the drawback of positioning inaccuracy at circuit or trace exposure in the prior art.
Moreover, non-solderable material is peelably applied over a support member and attached to the core layer in this invention, since adhesion between the support member and non-solderable material is considerably small, thus the support member can be easily peeled away from the non-solderable material, without damaging the attachment of the non-solderable material to the core layer; and the support member can be repetitively used, thereby effectively reducing fabrication costs. Further, without using an aluminum film for sustaining non-solderable material in the prior art, short-circuiting problems caused by surface roughness of the aluminum film should not be concerned in this invention.
Instead of plasma etching technique, grinding/laser-etching and chemical-etching processes are adopted to form openings for exposing predetermined terminals of conductive traces in the invention, and thus, drawbacks generated by using the plasma etching technique can be avoided. For example, the non-solderable material can be optionally added with fillers, and allow the circuit board of this invention to be structured with flexible mechanical strength. Moreover, without using the plasma etching technique, conductive traces of this circuit board would not be plasma-etched as in the case of the prior art; thereby, electrical connection quality and production yield of the circuit board can be assured in this invention.
In another embodiment, non-solderable material is directly applied over a jig instead of an additional support member, thereby further reducing fabrication complexity and costs.
In a further embodiment, openings formed through non-solderable material allow terminals of conductive traces to be completely exposed to the openings; these openings are NSMD (non-solder mask define) openings. Unlike adjacent SMD openings being spaced apart by at least the distance of unexposed part of neighboring conductive traces in the prior art, spacing between adjacent NSMD openings can be further reduced by decreasing the intermediate non-solderable material; this allows pitch distance for bond pads or fingers to be desirably reduced, making the circuit board of this invention suitable used for accommodating high-density arrangement of solder balls, bumps or wires.